Integrated photonics is proving to be a very promising platform for quantum information processing. Micro ring resonators are becoming a key component of such systems as they have been shown to be effective as photon-pair sources by means of exploiting a materials nonlinearity for spontaneous parametric downconversion (SPDC) or spontaneous four wave mixing (SFWM).
Often, it is desirable to have precisely one photon. While SPDC and SFWM sources generate pairs of photons, single photons can be achieved through heralding. Heralding is a technique in which the detection of a single photon from a pair is used to determine the existence of the other. One of the fundamental issues with ring resonators is their inherent 50% loss when critically coupled, regardless of operation in a single bus or double bus configuration. For single bus resonators (not shown), half of the generated photons are lost to scattering within the cavity.
Referring to FIG. 1 depicts prior art double bus resonators which are slightly different as the photons are free to leave the ring 10 through either port—resulting in an effective loss of 50%. All of this assumes that the ring resonator is critically coupled to straight waveguides 20, 30.
As with the two typical forms of ring resonators, they are denoted by the number of waveguides which near them giving them the titles of single bus and double bus, respectively. Both resonators work on the same principle. When light after a full round trip around the ring is of equal intensity and opposite phase to light that is reflecting into the ring, there is a destructive interference and no light can leave the resonator. Running time in reverse and seeing the light from the ring split at the directional coupler is an equivalent way to view this effect. In the case of the single bus resonator with no loss, resonance can only happen for a coupling ratio of 50/50 from the bus waveguide. When loss is present, this can happen for much lower splitting ratios. One form that loss can take is scattering. The double bus resonator can be seen as a special case of the single bus resonator where the scattering is captured into the second waveguide.
When the ring resonator is used for generation of single photons, two pump photons are absorbed and two single photons of equal energy to the pumps are created. Consequentially, the single photon light which is generated inside of the cavity has no input light to interfere with. Still referring to FIG. 1, therefore, in the case of the double bus resonator with the same coupler on input and output, the light has an equal probability of exiting the first 20 and second 30 waveguide buses. This splitting is witnessed as intrinsic loss. In the case of single bus ring resonators, the light can either leave through the input port or be lost inside the ring. When the pump wavelengths are optimally coupled, the propagation losses around the ring balance with the coupling out of the ring. The generated single photons (like the pumps) will have this same balance in terms of loss and ability to couple out of the ring. In other words, the single photons leave the ring only 50% of the time. The odds of the single photons leaving the ring can be improved at the cost of how well the pump wavelengths are coupled. This is a compromise between loss and generation rate.
The underlying issue of single and double bus ring resonators is that they do not have wavelength discriminating couplers. It is well understood there doesn't exist dichroic mirrors on a chip presently. Moreover, in 1995, Barbarossa found that resonant wavelengths of a micro ring cavity could theoretically be suppressed by coupling the input waveguide to the ring at two points. However Barbarossa's design provided an optical filter for classical light without generating any photons in the resonator cavity. What is lacking in prior work and therefore still needed is a device that generates entangled pairs of photons and interferometric coupling as a filter for quantum states of light.